Advanced Extremely High Frequency

Overview

AEHF satellites employ numerous narrow spot beams directed at Earth to relay communications to and from users. Crosslinks between satellites enable direct relay of communications without requiring a ground station. The satellites are designed for jam-resistant communications with a low probability of interception, using frequency-hopping radio technology and phased array antennas that can adapt their radiation patterns to block potential jamming sources.

The system incorporates existing Milstar low data-rate signals (75–2400 bit/s) and medium data-rate signals (4.8 kbit/s–1.544 Mbit/s), as well as a new signal supporting data rates up to 8.192 Mbit/s. The complete space segment of six satellites provides coverage of Earth's surface between latitudes 65° north and 65° south; the Enhanced Polar System supplements AEHF for northern polar regions. The initial design and development contract was awarded to Lockheed Martin Space Systems and Northrop Grumman Space Technology in November 2001, covering construction and launch of three satellites and a mission control segment.

AEHF is operated by the 4th Space Operations Squadron at Schriever Space Force Base and the 148th Space Operations Squadron at Vandenberg SFB. The system extends cross-links from earlier Milstar satellites, reducing vulnerability to attacks on ground stations. In April 2009, Secretary of Defense Robert Gates planned to cancel the Transformational Satellite Communications System in favor of additional AEHF capacity.

Electronics

This satellite uses extremely high frequency for sending signals up and for links between satellites, while the downlinks to Earth operate in the super high frequency band. To handle this mix and keep downlinks tightly focused for security, the spacecraft carries a varied set of antennas: two SHF downlink phased arrays, two crosslinks for satellite-to-satellite communication, two nulling antennas for uplink and downlink, one EHF uplink phased array, six gimbaled dish antennas, and one Earth coverage horn.

Phased array technology, a first for communications satellites, boosts reliability by doing away with moving parts. The low-gain Earth coverage horns can reach any point within a third of the planet covered by the satellite’s footprint. The phased arrays offer super high-gain coverage, giving all users—including small portable terminals and submarines—worldwide unscheduled access. Six medium-resolution spot antennas can be time-shared to cover up to 24 targets.

Two high-resolution antennas work even when a jammer is in the beam, and the nulling antennas help distinguish real signals from electronic attacks. Solid-state transmitters replace the older traveling wave tubes, allowing variable power output for lower interception risk and better efficiency. The flight software runs about 500,000 lines of real-time, distributed, embedded code across 25 onboard processors.

Services

AEHF can send individual digital data streams at speeds from 75 bits per second up to 8 megabits per second. These rates cover and surpass Milstar’s low, medium, and the submarine-specific high data rate, which is actually quite slow. The faster connections are called extended data rates.

For airborne use, the Family of Advanced Beyond Line-of-Sight-Terminal project handles the terminal, with Boeing as the lead contractor and L-3 Communications and Rockwell Collins as key partners. The first FAB-T unit, Increment 1, was delivered in February 2009 for the B-2 Spirit, and it is slated for other aircraft like the B-52, RC-135, E-4, and E-6, plus fixed and mobile command posts. Other ground terminals include the Single-Channel Antijam Man-Portable Terminal, the Secure Mobile Anti-jam Reliable Tactical Terminal, and the Submarine High Data Rate system.

Satellites

The first Advanced Extremely High Frequency satellite, designated USA-214, finally launched on August 14, 2010, from Cape Canaveral Air Force Station's Space Launch Complex 41 atop an Atlas V 531 rocket. This launch came four years late because the National Security Agency failed to deliver essential cryptographic equipment to the payload contractor on time. The Atlas V placed the satellite into a supersynchronous-apogee transfer orbit with a low point of 275 kilometers, a high point of 50,000 kilometers, and a 22.1-degree inclination.

The satellite's liquid apogee engine, built by IHI, could not raise the orbit after two tries. To work around this, twelve firings of smaller Aerojet Rocketdyne reaction engine thrusters, meant for attitude control during the main engine burns, lifted the perigee to 4,700 kilometers. From there, solar panels deployed, and over nine months, the orbit was raised using highly efficient but low-thrust 0.27 Newton Hall thrusters, also from Aerojet Rocketdyne.

A July 2011 Government Accountability Office report blamed the engine failure on a piece of cloth left in a fuel line during manufacturing, though a Department of Defense report also cited fuel loading issues and unmet thermal control requirements. The remaining satellites were declared flight-ready a month before that report. The second satellite, AEHF-2, launched on May 4, 2012, on the same rocket configuration from the same pad. After three months of maneuvering, it reached its operational position, and checkout was completed by November 14, 2012, when control transferred to the 14th Air Force for a planned 14-year mission ending in 2026.

Did You Know?

• The payload weight at launch is approximately 9000 kg; after expending propellants to achieve proper orbit, its weight is approximately 6168 kg.
• It takes over 100 days for orbital adjustments to reach a stable geo-position after launch.